Rather than seeing this keyword list as visible keywords for the article, Google's "define" operator algo took this list to be a definition of "keywords"... most probably because the word keywords was followed by a colon and words separated by commas in a dictionary-like form. This was the only instance of the term "keywords" on the page.
The page was in a directory called "/journal/" ...which might also have had an effect. Many of the other "keywords" definitions come from pages or directories named "glossary," "papers," "library," etc. So, there's a set of bookish references that Google might look for as well.
I also did the following search on Google...
site:webmasterworld.com google definitions
Among many others, the following threads, which looked promising, produced observations similar to what I'm noting above.
Getting listed as a Google Definition
How to get your site listed as a Google definition